CLINICAL DIFFERENTIATION OF SEPTIC ARTHRITIS AND OTHER ARTHROPATHIES

Participants: C. Craig, G. Krasan, J.M. Hall

Keywords: septic arthritis, transient synovitis, irritable hip

Introduction

Although septic arthritis and transient synovitis have similar clinical features, their respective clinical courses and their potential for negative sequelae are very different. Septic arthritis is associated with significant morbidity and mortality, treatment requires surgical drainage and antibiotics, and delayed diagnosis tends to result in poor outcomes, especially in children. In contrast, transient synovitis is usually self limiting, requires only symptomatic treatment and typically has a benign clinical course. Consequently, a valid and reliable method for differentiating septic arthritis and transient synovitis is essential.

Currently, differentiating between septic arthritis and other inflammatory conditions of the joint, such as transient synovitis, is a diagnostic challenge, the outcome of which is loosely based on an amalgam of clinical suspicion, initial laboratory values, clinical signs and radiographic presentation. However, a more precise method of diagnosis, based on those same measures, may be possible. From a retrospective review of cases evaluated at Harvard Medical School’s Children’s Hospital between 1979 and 1996, Kocher¹ developed and tested an evidence-based clinical prediction algorithm for differentiating septic arthritis from transient synovitis in pediatric patients with an acutely irritable hip. The algorithm predicts the probability of septic arthritis based on the number of multivariate predictors present (0 to 4). The four predictors are: history of fever, non-weight bearing status, erythrocyte sedimentation rate ³ 40 mm per hour, and serum WBC > 12x10⁹ cells/liter. The authors computed the probability of septic arthritis as 0.2 percent for patients with none of the four predictors, 3.0 percent for one predictor present, 40.0 percent for two predictors, 93.1 percent for three predictors and 99.6 percent for four predictors.

The specific aim of this study is to determine whether the application of the Evidence-Based Clinical Prediction Algorithm for septic arthritis published by Kocher et al¹ produces predictive probabilities in the UMMC pediatric irritable hip population that are similar to the Harvard study’s results.

Materials and Methods

Hospital discharge diagnosis data obtained from the Michigan Hospital Association and Michigan population statistics will be used to estimate the incidence of pediatric septic arthritis in the Michigan population. This incidence estimate will be used to compute statistical power once we determine how many patients in the hospital database meet inclusion criteria. A standardized electronic form (or paper version of the same) will be used to abstract data from medical records of pediatric patients who presented with an acutely irritable hip between 1990 and 2000. Cases will be categorized as true SA, presumed SA and transient synovitis according to the diagnostic criteria used by the Harvard group. A multivariate general linear model will be used to compare demographic, clinical, laboratory, and radiographic measures across the diagnostic groups described above. Variables with a p value less than 0.20 will be included in the logistic regression model. Backward Stepwise Logistic Regression will be used to determine whether the same group of independent clinical predictors identified in the Kocher et al study are statistically significant in the UMMC patient sample. Between group comparisons will be conducted for the groups defined above. Regression model fit will be estimated with the Hosmer-Lemeshow goodness-of-fit test. Adjusted odds ratios and 95 percent confidence intervals will be estimated using the method of maximum likelihood. The probability of septic arthritis will be estimated for each combination of predictors. Sensitivity and specificity of the final algorithm will be determined using standard formulae.

Progress

IRB approval has been obtained for this study and data are being abstracted from the medical records.

References

[1] Kocher M, Zurakowski D, Kasser J 1999 Differentiating Between Septic Arthritis and Transient Synovitis of the Hip in Children. The Journal of Bone and Joint Surgery 81-A(12):1662-1670